The present invention relates to a control system for an automatic transmission for a wheeled vehicle, such as an automotive vehicle, and more particularly to a control system for an automatic transmission which is designed to alleviate shift shock occurring upon a change in operational state of a one-way clutch during a shift operation.
Laid-open Japanese patent application No. 52-106064 (corresponding to U.S. Pat. No. 4,031,782 issued to Miller et al. on June 28, 1977) discloses a control system for an automatic transmission. This known control system comprises a closed loop controller, and a torque sensor positioned to sense the torque at the transmission output shaft to provide a torque indicative signal. The torque indicative signal is supplied to the closed loop controller which is operative to regulate actuating hydraulic pressure with good precision in response to the torque indicative signal. This control system is therefore directed to the precision control of hydraulic pressure applied to a friction element which is to be engaged during shift operation. Laid-open Japanese patent application No. 53-85264 (corresponding to U.S. Pat. No. 4,102,222 issued to Miller et al. on July 25, 1978) discloses an electronic control system which is directed not only to a precision control of hydraulic pressure applied to a friction element which is to be engaged during a shift operation, but also to a precision control of hydraulic pressure applied to another friction element which is to be released during the shift operation. According to this known control system, a torque sensor senses the torque at the transmission output shaft and provides an output shaft torque indicative signal. A first regulator valve regulates the hydraulic pressure applied to the friction element which is to be engaged during a shift operation, and a second control valve regulates the hydraulic fluid pressure applied to the another friction element which is to be released during the shift operation. An error signal is provided to control the operation of each of the first and second regulator valves. A closed loop controller is operatively coupled with the torque sensor and also with both of the first and second control valves and it receives the output torque indicative signal and generates signals as a function of the output torque indicative signal for controlling the first and second control valves.
In each of the known control systems mentioned above, a torque sensor is positioned to sense the torque at the transmission output shaft and provides an output shaft torque indicative signal. The output shaft torque indicative signal is fed to a closed loop controller which computes an error from a target value and generates a control signal for controlling a regulator valve connected to a friction element such that the output shaft torque varies in a predetermined desired pattern, thus alleviating or avoiding substantial shift shocks.
If the feedback control employed by the above mentioned control systems is to be carried out by a control system using a microcomputer, a microcomputer capable of high speed operation is needed. Such a microcomputer is relatively expensive.
If the output signal of the torque sensor is interferred with by error components, such as noise, this causes a drop in control precision of the whole system immediately since this is a realtime control. Thus, in order to prevent such a drop in control precision, relatively expensive torque sensor that provides noise free performance is required.
The present invention aims at solving the above mentioned problems encountered in the known control systems.
An object of the present invention, therefore, is to provide a control system for an automatic transmission which does not require a torque sensor with high precision performance nor a closed loop controller for controlling actuating hydraulic pressure applied to a friction element.